1. Field of the Invention
The present invention relates to a solid-electrolyte battery incorporating a wound electrode constituted by, in a lengthwise direction of the laminate, winding elongated positive electrode and negative electrode laminated to sandwich a solid electrolyte and a manufacturing method therefor.
2. Description of the Related Art
In recent years, a multiplicity of pieces of portable electronic apparatus, such as camcoders, portable telephones and portable computers have been marketed. There is a requirement for reduction in the size and weight of the portable electronic apparatus. Also reduction in the size and weight of a battery serving as a portable power source for the electronic apparatus is required. To meet the requirement, a lithium ion battery has been developed and put into practical use. The lithium ion battery is structured such that an ion conducting member disposed between the positive electrode and the negative electrode incorporates a porous polymer separator impregnated with electrolyte solution. To prevent leakage of the electrolyte, the overall body of the battery is packaged in a heavy and thick metal container.
On the other hand, reduction in the size and weight of a solid-electrolyte battery is expected which is structured such that the solid electrolyte is constituted by an ion conducting member between the positive electrode and the negative electrode and free of leakage of the solution. In particular, a solid polymer electrolyte and a gel-like solid electrolyte (hereinafter called a “gel electrolyte”), containing an electrolytic solution in a matrix polymer, is attracting attention.
The gel-electrolyte battery incorporating the gel electrolyte can be manufactured as follows.
The positive electrode is manufactured by uniformly applying a positive-electrode mix containing a positive-electrode active material, a conductive material and a binder to the two sides of a positive-electrode collector. Then, the positive electrode mix is dried so that a positive-electrode active material layer is formed. Then, the layer is dried, and then a roll press is operated so that a positive electrode sheet is obtained.
The negative electrode is manufactured by uniformly applying a negative-electrode mix containing a negative-electrode active material and a binder to the two sides of a negative-electrode collector. Then, the negative-electrode mix is dried so that a negative-electrode active material layer is formed. Then, the layer is dried, and then a roll process is operated so that a negative electrode sheet is obtained.
The gel electrolyte layer is manufactured by uniformly applying a sol electrolyte solution containing nonacqueous solvent, a salt of an electrolyte and matrix polymers to the two sides of the positive electrode sheet and the negative electrode sheet. Then, the solution is dried so that the solvent is removed. Thus, the gel electrolyte layer is formed on each of the positive-electrode active material layer and the negative-electrode active material layer.
Then, the positive electrode sheet on which the gel electrolyte layer has been formed is cut into an elongated shape. Then, the gel electrolyte layer and the positive-electrode active material layer in the portion in which the positive electrode lead will be welded are removed by cutting. Then, the positive electrode lead is welded to the cut portion so that an elongated positive electrode having the gel electrolyte layer formed thereon is manufactured.
Then, the negative electrode sheet on which the gel electrolyte layer has been formed is cut into an elongated shape. Then, the gel electrolyte layer and the negative-electrode active material layer in the portion in which the negative electrode lead will be welded are removed by cutting. Then, the negative electrode lead is welded to the cut portion so that an elongated negative electrode having the gel electrolyte layer formed thereon is manufactured.
A final process is performed such that the elongated positive electrode and the elongated negative electrode each having the gel electrolyte layer are laminated. The laminate is wound in its lengthwise direction many times so that a wound electrode can be obtained. The wound electrode is sandwiched in a packaging film. The outermost end of the packaging film is welded with heat under reduced pressure so that the opening portions are closed. Then, the wound electrode is hermetically enclosed in the packaging film so that the gel electrolyte battery is manufactured.
The gel electrolyte battery incorporating the wound electrode suffers from a low energy density and unsatisfactory heavy-load resistance. What is worse, there arises another problem in that lithium is deposited on the negative electrode.
The reason for this will now be described. Since the gel electrolyte layer formed on the positive electrode and that formed on the negative electrode are not integrated with each other, portions in each of which the gel electrolyte layers cannot be brought into hermetic contact with each other exist. If the portion in which the gel electrolyte layers cannot be brought into hermetic contact with each other exists, doping of lithium ions into the negative electrode, which is performed when a charging operation is performed, is inhibited.
When lithium ions cannot be doped into the negative electrode, a designed discharge capacity cannot be obtained. Thus, the energy density is lowered. Moreover, excessively high internal resistance of the battery causes the heavy-load resistance to deteriorate. In the portions in each of which the gel electrolyte layers cannot be brought into hermetic contact with each other, doping of lithium ions into the negative electrode is not performed when the charging operation is performed. As an alternative to this, growth of dendrite of lithium undesirably takes place from the negative electrode in the portions adjacent to the foregoing portions. The dendrite projects over the gel electrolyte layer. Thus, there is apprehension that minor short circuit occurs.